This is a phase II, randomized, controlled trial of combination antiretroviral therapy to assess efficacy and safety of 4 regimens. Enrollees will be HIV-1-infected with prior antiretroviral experience, including 1-3 protease inhibitors (PIs), and have HIV RNA >1000 copies/mL on their current therapy. The treatment groups are: Group A: amprenavir (APV), saquinavir (SQV), abacavir (ABC), efavirenz (EFV) and adefovir dipivoxil (ADV); Group: APV, indinavir (IDV), ABC, EFV and ADV; Group C: APV, nelfinavir (NFV), ABC, EFV and ADV; Group D: APV, (protease inhibitor placebo), ABC, EFV and ADV. All subjects will receive L-carnitine supplementation (to offset adefovir's effect on carnitine). A total of 460 subjects nationwide and 10-20 locally will be enrolled. The CRC is essential for the Pharmacokinetic substudy, which is mandatory for the first 10 patients in each arm, and will also enroll 40 patients with virologic failure and 40 controls. This substudy will do complete pharmacokinetic profiles on day 14 for APV, EFV, ADV, and for SQV, IDV, or NFV in relevant subjects.